Multiple sclerosis (MS) is a devastating neurodegenerative disease, characterized by chronic inflammation and demyelination. The incidence of MS is 2-3 times higher in women. However, the relapse rate of MS decreases during late pregnancy and also after treatment with pregnancy levels of estriol (a form of estrogen), leading to a decrease in CNS lesions. Estrogen (E2) is a potent regulator of the immune system and may also act directly on cells of the CNS, including microglia, astrocytes, oligodendrocytes and neurons. Our laboratory has convincingly demonstrated that estrogens exert a pronounced protective effect on clinical and histological disease in the animal model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE). Immunoregulatory properties of estrogen include dampening proinflammatory cells (e.g. dendritic cells, macrophages and encephalitogenic T cells) and activating Breg and Treg cells. Our goal is to determine the immune-mediated mechanisms that lead to protection of CNS cells (e.g. neurons, oligodendrocytes and microglia, MG). Deciphering the neuroprotective and immunoregulatory effects of estrogen is important for its possible clinical application. Our recent findings demonstrate a requirement for B-cells in E2-mediated protection against EAE involving direct E2 effects on Breg cells mediated through ER and the PD-1/PD-L negative co-inhibitory pathway. It is likely that chronically activated microglia cause the neuronal and axonal degeneration that occurs in progressive forms of MS. Since the cause of this chronic microglial activation is uncertain, we propose in this application that MG cells represent one major target for regulatory B-cells and other B-cell subsets, whether by secretion of IL-10 or via direct means (cell-cell PD-1/PD-L signaling). In summary, this proposal will contribute new information regarding the potential role of E2 on B-cell subsets in regulating MG activation and protection against EAE.